WIP: Experimental support for Unitful-like statically-typed units

[MilesCranmer]

I understand this goes against the main point of this package, but it turned out to be extremely easy to stick in a StaticDimensions interface.

The trick is to store a regular dynamic Dimensions object (immutable struct) as one of the type parameters:

struct StaticDimensions{R,D<:AbstractDimensions{R},dim} <: AbstractStaticDimensions{R,D,dim}
    # (No fields)

    StaticDimensions(d::AbstractDimensions) = new{eltype(d),typeof(d),d}()
end

Promotion rules work as you'd expect with Unitful:

julia> using DynamicQuantities; using DynamicQuantities: StaticDimensions

julia> x = Quantity{Float64,StaticDimensions}.([1.0u"m", 1.0u"km", 10us"Constants.Mpc"])
3-element Vector{Quantity{Float64, StaticDimensions(m::Dimensions{FixedRational{Int32, 25200}})}}:
                  1.0 m
               1000.0 m
 3.085677581491367e23 m

Where you can see it correctly promotes the dimensions (even symbolic dimensions).

We can see that units can be inferred:

julia> x = Quantity{Float64,StaticDimensions}(10u"km")
10000.0 m

julia> y = Quantity{Float64,StaticDimensions}(1u"h")
3600.0 s

julia> @inferred x / y
2.7777777777777777 m s⁻¹

julia> typeof(x / y)
Quantity{Float64, StaticDimensions(m s⁻¹::Dimensions{FixedRational{Int32, 25200}})}

---

If anyone is interested, would love help in making this infer as well as Unitful. It then would make it really easy to switch between static and dynamic quantities – Dimensions for initial work, and then StaticDimensions whenever you are faced with optimizing a fast SIMD loop.

Any of @YingboMa @ChrisRackauckas @devmotion @aplavin, would you be up to help with improving this? I think the basics are ready, we would just need to ensure that inference is as good as Unitful.